Dual property steel armor

ABSTRACT

DISCLOSED IS A DUAL PROPERTY STEEL ARMOR MATERIAL IN THE FORM OF A COMPOSITE, ROLL-BONDED STRUCTURE HAVING AN OUTER OR IMPACT LAYER OF AN IMPROVED ALLOY OF STEEL CAPABLE OF SHATTERING HARDENED STEEL ARMOR PIERCING PROJECTILES AND A HIGH TOUGHNESS BACKING LAYER OF AN IMPROVED ALLOY OF STEEL TO ACHIEVE MULTIPLE STRIKE INTEGRITY. THE OUTER OR IMPACT LAYER IS A LOW CARBON STEEL ALLOY CONTAINING THE CARBIDE FORMING ELEMENTS CHROMIUM, MOLYBDENUM, AND VANADIUM ALONG WITH MANGANESE, SILICON, AND NICKEL. THE HIGH TOUGHNESS BACKING LAYER IS COMPRISED OF A LOWER CARBON CONTENT OF A LOW CARBON STEEL ALLOY THAN STEEL ALLOY USED FOR OUTER IMPACT MATERIAL. THE COMPOSITE STRUCTURE IS PROVIDED WITH IMPROVED BALLISTIC PERFORMANCE THROUGH THERMAL-MECHANICAL PROCESSING OF A PROPERLY CONSTITUTED ALLOY STSTEM.

U.S.' Cl: 2 9196.1

United States Patent Oflice 3,694,174 Patented Sept. 26, 1972 'jBill N. Briggs, Santa Ana, Calif., assignor to the United States of America as represented by the Secretary of the Army No Drawing. Filed May 13, 1971, Ser. No. 143,233

' Int. Cl. B32b 15/00 4 Claims ABSTRACT on THE DISCLOSURE Disclosed is a dual property steel armor material in the formcfa composite, roll-bonded structure having an outer or Impact layer of an improved alloy of steel capable of shattering hardened steel armor piercing projectiles and a high toughness backing layer of an improved alloy of steel to achieve multiple strike integrity. The outer 'or Impact layer is a low carbon steel alloy containing the carbide forming elements chromium, molybdenum, and

vanadium along with manganese, silicon, and nickel. The

BACKGROUND OF THE INVENTION Evaluation of many concepts during the past several years has established that defeat of armor piercing projectiles is most effectively accomplished by projectile breakup. Steels of sufficient hardness to accomplish this, however, were likewise found to shatter.

Lightweight armors, i.e., having an areal density of less than 50% of rolled homogeneous steel, have been of particular interest. Such lightweight armors have also included composite materials B C/ woven roving fiberglass and dual property steel armor.

The dual property hardness steel armor has several distinct advantages over earlier prior art armor; such advantages include having requirements conducive to unlimited production quantities using existing facilities and having fabricability and intrinsic properties of steel. The earler concept for dual property steel armor was developed from the knowledge that a high hardness was needed to shatter steel armor piercing projectiles and a high toughness was required to achieve multiple strike integrity.

Hot-cold working or thermal-mechanical processing has been employed to produce steel panels havin improved strength and toughness in properly constituted ste'els. Basically, the process consists of plastic deformation of a high temperature austenitic structure of steel followed by cooling to room temperature at a suitable rate necessary to form martensite and then tempering to :the desired strength and toughness levels. Thus, the described process in general differs from the conventional fabrication of alloy steel components in that the plastic deformation is done as a part of the heat treat cycle. The characteristics of the individual alloy composites establish the actual details of the processing; austenitizing time and temperature, cooling rates, forming temperatures, etc.

Although the dual property steel armor principle pro- .vides an alloy capable of breaking up the projectile,

numerous tested alloys have resulted in panel shattering. When panel shattering occurs the elfectiveness of the armor is lost, particularly, as an armor suitable for a multiple strike capability.

tough backing material capable of stopping the broken up projectile'Also, desirable would be a steel having lower alloy content and a lower production cost than currently employed armor material. The alloy should have sufficient hardenability for the impact or front face and backup or backface when processed under the same processing parameters. The alloy armor front and back face materials must be metallurgically compatible, i.e., achieve optimum properties under the same processing parameters.

An object of this invention is to provide a dual property steel alloy having a composition for use as the impact face and having a composition for use as the backup face which are metallurgically compatible and capable of being processed conjointly under the same processing parameters.

Another object of this invention is to provide a dual property steel armor which can be fabricated from. a preform having two distinct steel compositions which are metallurgically compatible for processing under same processing parameters to yield a high hardness impact layer and a lower hardness (i.e., about 5 to about 8 R lower), substantially tougher backing layer.

SUMMARY OF THE INVENTION It has been discovered that the steel alloys, set forth below in Table I, when processed in accordance with a known roll-bonded technique employed in conjunction with the disclosed austenitizing and tempering temperatures, yield a high quality dual property steel armor.

TABLE I.NOMINAL COMPOSITION Percent Alloy 0 Mn Si Ni Cr Mo V D-ll 0. 44 0. 72 0. 40 0. 60 1. 00 2. 00 0. 5O ADPX-27 0. 28 0. 65 0. 25 3. 25 l. 00 2. 00 0. 10

. TABLE II.-NOMINAL COMPOSITIONS Percent Alloy Gr Mo Si 0 Ni DESCRIPTION OF THE PREFERRED EMBODIMENT The preferred steel alloy employed in the front face of the armor of this invention has a nominal composition as follows:

ALLOY D-ll Nominal composition, percent C 0.44 Mn 0.72 Si 0.40 Ni 0.60 Cr. 1.00 Mo 2.00 V 0.50

' The preferred steel alloy employed in the back face of the armor-of'thisinvention-has a-nominal composition as follows:

ALLOY ADPX-27 Rolling of 6" x 6 x 2" performs can be performed on a -----12" x 12" -2-high rolling mill powered by a 50 HP- AC Nominal composition, percent C v 0.28 M 0.65 Si I 0.25 i 3.25 C 1.00 M 2.00 V 0.10

The above steel alloys may be made by electric furnace or other commonsteel making practices..The steel alloys were prepared by consumable arc vacuum melt techniques. v I

L The starting materialsfor making armor test components comprised of the above. steel alloys are preforms which consist of a prepared matching sheet of the impact ,alloy of steel and a prepared matching sheet of the backupalloy'which are ground to a 32 RMS finish on the surfaces to be bondedand beveled "-45 on the edges towbe seal welded. Cleaning is then performed as follows:

(1) Degrease in Trichlor (2) Scrub with pumice-cleanser (3) Wash with distilled water (4) Rinse in acohol (5) Air dry After cleaning the components were assembled and either placed in a vacuum box for storage or weldedwithin the same eight-hour shift. Electron beam seal welding was performed on a KV Sciaky electron-beam welder which was equipped with an automatic wire feed. Preforms were welded along the longer side, flipped and welded on the reverse side. Each end was then welded in turn. The small performs were usually welded'in lots of four to six. After welding one seam of each preform the chamber would be opened and the preforms turned. .The

welding parameters are give below:

Weld current-19 kv.

Weld amperage-440 ma. Speed20 i.p.m.

Wire Ty-pe-316L 0.035" diameter Wire feed rate-l00 -i.p.m. Vacuum'-10- torr The welded preforms are thermal-mechanically procthickness of preform during hot rolling from 2.250 to 0.600". Consequently, reheating is necessary. The preforms are preheated. at 1700 F. for 45 minutes in an endothermic atmosphere (46 F. dewpoint), transferred to an air furnace, and held for 35 minutes at 2100 F., then rolled. Rollingtimewas 15-20 minutes including reheating time.

=Pieces are-then either air cooled or quenched in oil.

Subsequent to hot rolling of the preforms to form the roll bonded pieces, the roll bonded'pieces are sandblasted to remove scale, austenitized in an endothermic atmosphere at 1900 F. for l'hour, cooled to 1500 F., placed in a holding'furnace (air), held for 5 minutes, and then rolled continuously until a 50% reduction is achieved.

The pieces are then immediately oil quenched'and temp'ered for 2 hours. These processing parameters'are selected to minimize decarburization and achieve optimum properties.

drive. Here the pieces are charged directly into a 2100 F. air furnace for 1 hour, hot rolled from 2.00 to 0.600", and air cooled. Generally this operation is performed without reheating. These roll bonded preforms are subsequently austenitized at 1900 F. for 1 hour (air), cooled to 1300-1500 F., rolled continuously, oil quenched, and stress relieved at 200 F. Pieces are subsequently tempered for 2 hours at 400 F. v

Rolling of preforms of larger dimensions (e.g., 12" x24" x 3.5") is readily' accomplished with a Mesta 2- hi gh 4-high mill having 30" diameter backup rolls and 15" diameter work'rolls x 36" face. The mill is powered by a 600 HP DC drive-and designed to sustain aseparation force of 2,000,000 pounds. Preforms are preheated at '1500" F. in air for 1 hour then charged into a carbon tube induction furnace having .a protective atmosphere of nitrogen at 2200" F. for 2 hours. The 12" x 24" x 3.5" preforms are then hot rolled to' 0.600",.hot sheared, and air cooled. The pieces are subsequentlyaustenitized at 1900 F. for 1% hours, air cooled to 1400-1500 F., rolled continuously, oil quenched, and tempered for 2 hours in hot oil at 400 F.

Table III sets forth data to showthe effect to hardness of alloys D-ll and ADPX-27, the'impact alloy and backing alloy respectively, which have been tempered following the thermalmechanical'pro'eessirig.

1 Indicates range of hardness, Rockwell 0 scale, of a majority. of samples tested.

Numerous test samples when tempered at 400 F. for 2'hours yield hardnesses for the D-ll/ADPX-27 in the range of 59/53 to 5 8/ 52. Other treatment completed prior to oil tempering included austenitizing at "1900 F. lhrL/ inch and thermal-mechanically processing to 50% reduction over a falling temperature range of approximately 1500-1200 F.

Armor plate made using a bondedimpact face of -D-1"1 alloy and a backup face of -ADPX-27 alloy'when ballistic testedand compared with ballistic tests made with prior "art alloy-of H-ll/HP 9-4-30 showed an improvement surpassing 10%, respective to merit ratings obtained. The improvement is attributed in'part' to adequate nickel-content in the backup layer to alfect toughness of the backing material,"and additionally, to the chromium content which significantly reduces decarburization and scale. Surface grinding offers significant increase in ballisticpe'rformance on some composites; however, the alloy chemistry is shown to be very influential on both decarburization and scale.

I claim: 1

1. -A dual property steel armor material in the formof a composite, roll-bonded structure processed bythermalmechanical processing, said structure having an outer impact layer 'of' an alloy of steel and abacking layer of an alloy of'steel; said outer impact layer of an alloy of. steel characterized by having a nominal chemical composition "in percent by weight of about: 0.44 carbon, 0.72 manganese, 0.40 silicon, 0.60 nickel, 1.00 chromium, 2.00molybdenum, and 0.50 vanadium, said backing layer of an alloy of steel characterized by having a nominal chemical composition in percent by weight of about: 0.28 carbon, 0.65 manganese, 0.25 silicon, 2.00 to about 3.25 nickel, 1.00 chromium, 2.00 molybdenum, and 0.10 vanadium; and said outer impact layer of an alloy of steel and said backing layer of an alloy of steel being metallurgically compatible for processing under same processing parameters to yield a high hardness impact layer and a lower hardness, substantially tougher backing layer.

2. The dual property steel armor material in the form of a composite, roll-bonded structure as set forth in claim 1 wherein said structure is rolled-bonded from a degreased and cleaned seal-welded preform that is prepared from a matching sheet of said impact layer of an alloy of steel and a matching sheet of said backing layer of an alloy of steel and wherein said preform is processed by the thermal-mechanical processing which comprises the steps of:

(a) preheating said preform to a predetermined temperature in the range of about 1500 F. in air for a predetermined period of time of about 1 hour;

(b) transferring said preheated preform to a furnace having a protective atmosphere of nitrogen at about 2100 to about 2200 F. and holding said preform for a predetermined period of time between about 30 minutes and about 2 hours; then,

(c) hot rolling said preheated perform to accomplish up to about 70% reduction in thickness;

(d) cooling said rolled preform and removing scale;

(e) austentizing said rolled preform in an endothermic atmosphere at about 1900" F. for about 1 hour/inch thickness;

(f) air cooling austenitized perform to about 1500 F.

and holding for about 5 minutes in a suitable air furnace;

(g) rolling cooled preform continuously until about a reduction in thickness is achieved and immediately oil quenching; and

(h) tempering the rolled and quenched preform for about 2 hours at a selected tempering temperature of from about 400 F; to about 900 F.

3. The dual property steel armor material in the form of a composite, roll-bonded structure as set forth in claim 2 wherein said nominal chemical composition of nickel is about 3.25 percent by weight in said backing layer of an alloy of steel and said tempering is accomplished at about 400 F. to yield a dual property steel armor material characterized by having a measurable Rockwell C scale hardness of said impact layer of about 58 to about 59 and by having a measurable Rockwell C scale hardness of said backing layer of about 52 to about 53.

4. The dual property steel armor material in the form of a composite, roll-bonded structure as set forth in claim 2 wherein said nominal chemical composition of nickel is about 3.25 percent by weight in said backing layer of an alloy of steel and said tempering is accomplished at about 900 F. to yield a dual property steel armor material characterized by having a measurable Rockwell C scale hardness of said impact layer of about 55 to about 57 and by having a measurable Rockwell C scale hardness of said backing layer of about 50 to about 52.

References Cited UNITED STATES PATENTS 2,249,629 7/1941 Hopkins 29l96.1 2,391,353 12/1945 Sheridan 29-196.1 2,473,686 6/1949 Keene 29-196.1 2,562,467 7/1951 Kinnear 29-196.1 2,653,117 9/1953 Keene 29l96.1 2,881,109 4/1959 Thorn 29196.1

HYLAND BIZOT, Primary Examiner 

